Assertion: There is a demand for a 'plebiscite' in Kashmir.
Reason: The people feel that the promise that Accession would be referred to the people of the State after the situation created by tribal invasion was normalized, has not been fulfilled.

Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

In October 1947, Pakistan sent tribal infiltrators from its side to capture Kashmir. This forced the Maharaja to ask for Indian military help. India extended the military support and drove back the infiltrators from Kashmir valley, but only after the Maharaja had signed an ‘Instrument of Accession’ with the Government of India. The Union Nations Organisation in its resolution dated 21 April 1948 recommended a three step process to resolve the issue. Firstly, Pakistan had to withdraw its entire nationalities, who entered into Kashmir. Secondly, India needed to progressively reduce its forces so as to maintain law and order. Thirdly, a plebiscite was to be conducted in a free and impartial manner. However, no progress could be achieved under this resolution.

Internally, there is a dispute about the status of Kashmir within Indian union.  Articles 370 and 371 gave special status to Jammu and Kashmir. This special status provoked two opposite reactions. There is a section of people outside of J&K that believed that the special status of the State conferred by Article 370 did not allow full integration of the State with India. This section felt that Article 370 be revoked and J&K be treated like any other state of India. Another section, mostly Kashmiris, believe that the autonomy conferred by Article 370 is not enough. They had at least three major grievances. First, the promise that Accession would be referred to the people of the State after the situation created by tribal invasion was normalised, has not been fulfilled. This generated the demand for a plebiscite. Secondly, there was a feeling that the special federal status guaranteed by Article 370, had been eroded in practice. This had led to the demand for restoration of autonomy or ‘Greater State Autonomy’. Thirdly, it was felt that democracy which is practiced in the rest of India has not been similarly institutionalised in the State of Jammu and Kashmir.